This application claims the priority benefit of Taiwan application serial no. 90109977, filed on Apr. 26, 2001.
1. Field of Invention
The present invention relates to optical proximity correction. More particularly, the present invention relates to optical proximity correction using a rectangular contact having an increased y-x ratio.
2. Description of Related Art
As the level of integration in integrated circuits continues to increase, overall dimension of each circuit device is actually decreasing. Photolithographic process is among the most critical steps in the fabrication of quality integrated circuits. Most structures associated with a metal-oxide-semiconductor (MOS) device such as various thin film and doped regions involve a photolithographic process. In addition, further reduction of device line width so that a smaller critical dimension (CD) is obtained is very much dependent on the future development of photolithographic techniques. To meet increasingly stringent conditions, methods for increasing the resolution of a photomask keep developing. For example, optical proximity correction (OPC) and phase shift mask (PSM) are both means of increasing the critical dimension.
Optical proximity correction (OPC) is a method of reducing the proximity effect that can lead to a deviation of critical dimension (CD). The so-called proximity effect includes two related phenomena. When a light beam passes through a photomask, the light beam is diffracted and eventually causes an outward spreading. In addition, after the light beam reaches the surface of a wafer, the semiconductor substrate of the wafer reflects a portion of the incoming light back to result in some light interference. Consequently, double-exposure may occur, resulting in the over-exposure of a photoresist layer in some areas. The seriousness of such photoresist over-exposure is especially intense when the line width of a device is decreased.
A conventional optical proximity correction method can be used to improve the shape of a contact hole. FIG. 1A is a top view of a conventional optical proximity correction layout on a photomask for producing a contact. FIG. 1B is a view of the profile after exposure using the photomask shown in FIG. 1A. As shown in FIG. 1A, when the contact hole is designed to be circular, the OPC layout of the contact hole is usually a square pattern 100. As shown in FIG. 1B, the square OPC layout pattern 100 in the photomask forms a circular contact hole 102 after photo-exposure.
Although a conventional optical proximity correction that utilizes an OPC layout pattern is capable of improving contact hole profile, producing a rectangular contact pattern by the OPC method is difficult. Hence, a pattern having an optimal y-x ratio is difficult to obtain.
Accordingly, one object of the present invention is to provide an optical proximity correction for a rectangular contact capable of having a larger aspect ratio.
To achieve these and other advantages and in accordance with the purpose of the invention, the invention provides an optical proximity correction for rectangular contacts. The method involves combining two connected mirror-image hammerhead patterns and a plurality of serif patterns at the inner straight corners of the hammerhead patterns. By controlling the width of the connecting section between the hammerhead patterns, an optimal aspect ratio for the rectangular pattern is obtained. Furthermore, the actual pattern produced after photo-exposure is closer to a rectangular pattern as width of the connecting section between the pair of hammerheads is reduced.
As embodied and described herein, the invention provides an optical proximity correction method for producing a rectangular contact. The method begins by providing a first photomask with a first pattern formed thereon, wherein the first pattern has a pair of connected hammerhead patterns and a plurality of serif patterns at inner straight corners of the hammerhead patterns, with the hammerhead patterns being mirror images of each other. A first photolithographic step is performed for a first photoresist layer using the first photomask, so that a pattern correction for the first photomask is complete if the pattern on the first photoresist layer conforms to an aspect ratio of a desired rectangular pattern. Otherwise, a width of the connecting section between the hammerhead patterns in the first pattern is reduced for forming a second pattern on a second photomask. A second photolithographic step is then performed for a second photoresist layer using the second photomask. Next, the width of the connecting section is adjusted and a patterned photoresist layer is produced until a pattern on the photoresist layer conforms to the aspect ratio of the desired rectangular pattern.
In this invention, the rectangular contact pattern required by an integrated circuit is formed through a pair of opposing hammerhead pattern and a group of serif patterns. This type of optical proximity correction has greater precision for forming a rectangular contact having a larger aspect ratio than a conventional contact hold model. A wafer undergoing a photolithographic process is more capable of attaining the standard required by a product. Ultimately, both product yield and product quality is improved.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.